So far, in their journey of walking with monsters, the group witness their earliest fish ancestors escaped their armored enemies, the arthropods. They evolved legs and lungs and took their first steps on to land. But not far enough from the perils of the water.

Now their amphibian ancestors have made another huge leap forward. They've evolved into the first reptiles

But as they move inland, they'll face an ancient enemy. More deadly than ever before. The Arthropods are back.

300 Million Years Ago

Carboniferous Period

Place: Kansas, USA

Oxygen Content: 40% Above Today

Hazards: Giant Insects

Twilight and the others have been drop off at their new destination, the swamps of the Carboniferous.

300 Million Years Ago, much of the earth was covered in vast swamp forests. Here, nothing is as it seems. The 190 foot giants towering above this water world may look like trees, but they're actually distant relatives of ferns. Even the air is alien. Over a third is Oxygen. Much higher than at any other time in history.

Just look at this amazing place. Twilight amazed by the seen.

I'll say I'm getting mud on my hooves. Rarity complained.

They have been walking through some mud for a while before finally walking on solid ground.

Hey, it is me of is it a lot easier to breath now. Rainbow said noticing something.

Twilight had a idea using her magic, she made a screen appear showing different air levels. And Oxygen was the highest.

I don't believe it! Twilight said. The Oxygen level here is off the charts.

Twilight is right, Oxygen level in modern times are 21% but at this time it's 35% that's twice the amount back home.

So, that's why it's so easy to breath here. Spike said.

What is this place? Applejack asked.

I don't know. Starlight said. But it's almost our home.

The others had little idea how right Starlight was. But they are about to find out. This Oxygen rich Atmosphere has fueled the growth of new, super sized Arthropod predators.

Then they heard loud rustling in the ferns next to them.

What is that? Spike asked nervously.

I don't know. Pinkie said as they huddled close together in fear.

The rustle begins to get louder as what ever is was in the ferns got closer. But then it stop just a few feet from leaving the ferns, and they heard a rattling sound.

Was that a, rattlesnake? Rarity asked nervously.

No, it's sounds a softer than a rattler. Starlight said Unaware that snakes haven't arrived yet.

The group of 8 slowly back away from the ferns, and then suddenly a flash of black jumps out of the ferns making them jump a little. And the flash of black lands on the side of a tree and revealed itself. It was the biggest spider they have ever seen in their lives, much bigger than the Goliath birdeater spider of today. And for those with Arachnophobia, this is your worse nightmare.

This Mesothelae spider is the size of a Human's head. She'd be hunting anything the size of cats or even Spike if were alive today.

That is One Big Spider! Pinkie said a little scared.

Luckily the giant arachnid was not interested in the strange creatures that are near her home. She simply climbs downs, and crawls backwards into her burrow. Similar to a Trapdoor spider of today.

And she's an ambush predator. Operating from her underground headquarters. And inside the spider reaches the end of burrow which goes only almost 3 feet down. Safe in her burrow, she rests her feet on carefully constructed trip line. These allow her to pick up vibrations from the outside world.

What is it doing? Spike asked still scared by the spider's size.

I think it's just resting in her home. Twilight said.

Oh, I seen some spiders living in burrows back home. Fluttershy said.

But, I'm guessing never one that big. Starlight said.

Fluttershy nodded.

What's that piece of webbing for? Pinkie pointed to a silk thread a few feet away from the entrance.

I think it's a trip line of some kind. Twilight said. Some spiders do that to detect approaching prey.

Okay, let's just keep clear of the hole. Rainbow said.

As the gang continue to look at the hole carefully. Then a familiar animal from modern times appeared, a lizard. And unknowing to Spike his first scaly great grandfather.

Hey, look a lizard. Pinkie said.

Reptile Petrolacosaurus is hunting on the forest floor. Unlike their Amphibian ancestors, he has tough scaly skin which traps moisture inside his body. Vital for all land dwellers. Because he doesn't dry out in the sun, He can venture away from water. But that means encountering New Predators.

The Petrolacosaurus continues walking, but then Spike notice he is dangerously getting too close the spider's lair. And it wasn't long when the other notice and tried to warn him.

Inside, the Mesothelae can feel the vibrations as he got closer. And get's ready to strike and got in position.

Oh, I cant look. Fluttershy said covering her eyes.

As the reptile looks down in the hole, then in a sense of POP GOES THE SPIDER! The Mesothelae launches out of the burrow and the Petrolacosaurus turns and runs as fast as his legs can go. The spider begins to chase her target, but the lizard was keeping one step ahead of the arachnid. Twilight and the others follows the hunt til eventually The Petrolacosaurus ran inside a log and taking safety inside.

The reptile outruns the spider. He's evolved a complex heart, much more efficient than his enemy's. In fact the template for our own, as well for Twilight and her friends. It's a powerful pump that pushes blood and Oxygen around his body to his muscles, so he can run at high speed without stopping for a rest. This kind of heart also helps Rainbow Dash when flying that way she can fly at super high speeds without stopping for a rest for such a period of time.

This time though, he's trapped inside a log. And it wasn't long before the Mesothelae has caught up him as she is now at the entrance. The lizards turns to see her as she is trying to get in, by the entrance is too small for and the reptile backs away from her.

At first the spider's armor stops her from squeezing in. Outside Twilight and the others watch as the spider climbs on the log trying to find a way in.

But she's not going to give up so easily.

Back inside, the Petrolacosaurus continues to back away til he stops mid way. And as he stops he heard a rattling sound and sees a shadow, looking up he found himself looking at the spider squeezing through a gap that was big enough to squeeze through. Then she fell and lands right top of him before he even had the chance to react. The lizard screeches helplessly as the spider rears up and jabs him with her large fangs into him. And soon he was dead, Twilight and the others heard the screeches from outside and we're glad they didn't have to see it. And the spider begins to drag her meal back to her den.

Back in her lair, Mesothelae will inject her victim with digestive juices, to dissolve him from inside.

Not only the Arthropods have flourish in the swamps. The Carboniferous is one of Earth's greenest times. It's this very amount of plants, which has pump up the air's Oxygen to extremes. Fueling the Arthropods growth spurt.

The swamp's damp condition are perfect for plant growth. But the water levels are unpredictable.

They've risen.

Meanwhile back with Twilight and the others. They continue to follow the Mesothelae from a safe distance from her.

The spider drags her dinner home, but something's wrong.

As the mesothelae enters her home, but before she was completely in. She climb's out in goes in head first.

Inside she has discovered what has happen to her home when she was gone. Most of the hole has been filled with water.

Her headquarters are flooding. She'll have to evacuate.

She climb's back and drags her meal else where. Twilight and the others were confused of why she is leaving her home.

Where is it going? Spike asked.

I think there's something wrong with her burrow. Starlight said.

Rainbow took a careful look inside and saw water in it.

The hole has water in it. She said.

Oh, looks like our 8 legged friend is force to move. Fluttershy said feeling sorry.

I guess there's nothing wrong with following it. Twilight said.

The others begin to follow her from a safe distance. She can't dig, so she'll have to find another hole she can adapt. And once outside, she's vulnerable to larger predators.

As the spider drags her meal, suddenly out of nowhere a giant dragonfly snatches her dinner and takes off. Leaving the spider surprise and upset of losing her catch.

Uh, win some you lose some? Pinkie said confused.

What was that? Spike asked.

A big dragonfly. Rainbow Dash said.

Rainbow Dash was right. Meganeura is a monster Dragonfly, Queen of the Carboniferous skies. With a wingspan of almost 3 feet, she's the size of a eagle with an appetite to match.

Meanwhile the Meganeura has found a secure spot on top of a tree to feed.

High above her Carboniferous kingdom, normally she's invincible. But trouble's brewing.

Some distance away is a storm.

The flood below is due to an approaching storm. The super Oxygenated atmosphere, which has helped fuel the growth of the insects and arthropods, is highly volatile. A lightning strike could trigger an explosion.

The clock is ticking for the swamp.

Meanwhile down below, back with the others. They continue to follow the Mesothelae to the edge of a lake where she was scared off by a large amphibian called Proterogyrinus.

The rising water levels suit one group of hunters. Amphibians. They've continued thriving over the last 60 million years. Their thin skin still restrict them to the water's edge, but they're powerful predators with a devastating pair of jaws, ready to ambush anything that wanders within reach.

The gang continues to watch as the spider passes a patch of forest with several other burrows which have been taken.

The homeless spider tries her luck further inland. But other Mesothelae fill the forest floor, and they don't like to share.

As the spider and walks backwards, suddenly something came crawling out of the bushes behind as she quickly moves and turns to face the creature. Twilight and the others were stunned to see a giant millipede. And the millipede raise's haft it's body off the ground.

Neither does Arthropleura. His a distant relative of modern millipedes and centipedes, but as long as a car. He can rear up, tall enough to look at a person right in the eye. Although he's vegetarian, his strong jaws could still deliver a nasty bite.

Soon the Arthropleura drives off the spider, and he begin to crawl away as well.

With the rising waters, Arthropleura's foraging leads him into dangerous territory.

The Arthropleura wanders towards a lake, then a Protergyrinus confronts him as he climbs out of the water. And the giant millipede rears up. The 2 were soon in a stand off, almost like a Mongoose after a King Cobra. In this clash of the titans, the arthropod's armor is his best defense. Unless it gets split.

Then the amphibian overpowers's the Arthropleura and flips him over and hit a wooden spike. Within seconds the Arthropod is dead. Impaled, his soft innards provide a feast.

And the Proterogyrinus begins to feed.

Sometime later. The lightning storm is building, moving ever closer.

Meanwhile the group of 8 saw the mesothelae approaches a hole by a log.

Just in time, the spider finds another hole.

Then just where the spider was about to enter, a Petrolacosaurus came running out much of the spider's surprise, as well for Twilight in her friends. The Petrolacosaurus is evicted, but reptiles don't need burrows to avoid danger, they have speed and stamina on their side.

And the lizard disappears in the bushes. Fortunately for him, the Mesothelae is not interested or hungry.

Meanwhile the spider starts to turn the hole into a hunter's lair.

Hours later at Night falls.

And Mesothelae settles into her new silk lined home. She tests the new trip lines. They're already picking up vibrations from outside. But it's not Twilight and her friends who were nearby.

The storm is very close now. Lightning strikes ignite the Oxygen rich air.

Not far away giant dragonflies are forced down from the canopy to seek shelter below. An unexpected windfall for the water predator. As some dragonflies fly to close to some Protergyrinus trying to snatch them like Crocodiles catching bats in trees.

Meanwhile some Dragonflies rush pass above the spider's home. And Twilight and the others were getting concern.

Uh, guys maybe we should find some shelter. Twilight said.

Come on. Starlight said.

And begin to leave to find shelter.

Amid the chaos, Mesothelae seems safe in her bunker underground.

Then a bolt of lightning strikes.

Next morning reveals last night's storm.

A huge fire, ignited by lightning has devastated Mesothelae's neighborhood. Twilight and the others were lucky to escape with their lives.

There are no signs of life above ground.

As they look through the burnt area, Pinkie saw a familiar creature.

Hey look! Pinkie called to the others. It's one of those lizards.

However, a Petrolacosaurus has managed to outrun the flames.

Oh, thanks goodness something has got away. Fluttershy said.

Hey, does that hole look familiar. Spike said seeing a familiar looking hole.

Wait a second, that's the spider's home. He said.

They watch as the reptile moves closer. But now he heads right into the spider's lair. The Petrolacosaurus then walks right in, and the 8 friends waited for screeching. Fluttershy can't watch.

But strangely nothing happen.

Uh, maybe it's a trick. Rainbow Dash said.

Then suddenly the Petrolacosaurus came out of the hole completely unharmed. Much of their relieved, but the reptile has pulled something they wish for the same. As it was the burnt charred crisp body of the spider.

Mesothelae's hole was at the center of the lightning strike. But the reptile gets a spider barbecue.

The Era of the giant bugs is coming to an end. The climate is drying and the atmosphere is losing Oxygen. The monster insects and spiders can't survive the change.

But with their efficient hearts and waterproof skin, their versatile ancestors flourish in the new dry climate.

Evolution takes over.

Then the time warp opens once again and the gang are suck in taken to the next destination.

Lacking restrictive armor means the reptiles can start to make it big. Their muscles and bone broaden and extend. Their bodies mushroom into huge new shapes and forms. Their distant ancestors have come a long way. They're about to conquer the land.

Next Time, they'll travel forward another 20 million years to meet the first giant reptiles, their strange sailback ancestors who face their toughest enemy yet... Each other.

A/N that will be the only time I do something with spiders because I do not well with them.

Fun Fact: The mesothelae in Walking With Monsters was based on Megarachne which was once thought to be a spider. But which is really a sea scorpion.